1. Field of the Invention
The present invention relates to a power amplifier mainly for use in a device for mobile communication such as a portable telephone set and, more particularly, to a power amplifier capable of limiting a temperature dependence of the power gain at a specified power as well as a small-signal gain deviation.
2. Background Art
A power amplifier for mobile communication amplifies modulated waves output from a radiofrequency integrated circuit (RFIC) in a wireless band to a transmitted power value necessary for transmission from an antenna to a base station, a terminal or repeater. A maximum output voltage required of a power amplifier is specified for each mobile communication. A power amplification factor required of a power amplifier is a value sufficient for outputting the maximum output power of the power amplifier according to the maximum output power of an RFIC, and a minimum value of the power amplification factor is set by considering a loss due to a filter, an isolator or the like for spurious removal inserted between the output of the power amplifier and an antenna. On the other hand, there is a need to set the maximum output power of the power amplifier so that the levels of noise and spurious signals output from the power amplifier are not higher than a certain level.
Further, a high power efficiency and high linearity are required of a power amplifier. In ordinary cases, in order to increase the power efficiency of a power amplifier, the power amplifier is designed so that the maximum transmission power is closer to the saturation region of the power amplifier. However, the gain of the amplifier is reduced by a reduction in amplitude due to non-linearity in the vicinity of the saturation region. A distortion is simultaneously caused in the output signal due to the non-linearity. This becomes a cause of deterioration in adjacent channel leakage ratio (ACLR) and interference with other systems. Therefore, improving the linearity is of primary importance. The distortion can also be expressed by the amplitude input-output characteristic (AM-AM characteristic) of the power amplifier and the change in phase of the output signal (AM-PM characteristic). The power amplifier is designed so that each of the changes in characteristics is equal to or lower than a certain level in an output level region assumed with respect to the power amplifier.
In recent years, with the improvement in multifunctionality and the reduction in price of portable telephone sets, there has been a more and more increasing demand for reducing the manufacturing cost of power amplifiers for use in portable telephone sets. The development of power amplifiers using a CMOS process has become more active as a means for reducing the manufacturing cost.
With a power amplifier using a cascode amplifier, there is a problem that a change in power gain (ΔGp) at a specified output when the ambient temperature is changed is larger than that in a power amplifier using a source-grounded amplifier (see, for example, Japanese Patent Laid-Open No. 2013-98904). As a technique to limit a change in small-signal gain (ΔGl), adjustment of the bias point of a source-grounded transistor (idling current) with respect to a change in temperature is ordinarily performed.